Analog-to-digital converters (ADCs) allow for an analog input signal to be sampled into the digital domain. Common uses for ADCs include instrumentation, such as oscilloscopes, medical imaging, communications, and radar.
Electronic ADC's have made steady technological progress, but issues such as clock jitter and internal parameter mismatches make it difficult for ADCs to maintain high resolution as the sampling rate increases. It is common for ADC resolution to fall 1 bit for every factor of 2 rate increase [1].
Photonic technology can be used to aid in creating faster ADCs. The performance improvement is due to various factors depending on the specific design, but may stem, for instance, from the ability to generate ultra-short pulses with ultra-low timing jitter in the optical domain. Twichell [2] used an optical modulator with two optical outputs, detected and digitized the two outputs, then applied an inverse transformation via a digital signal processor (DSP) to re-construct the voltage applied to the modulator. A common modulator to use is a dual-output Mach-Zehnder modulator which interferes two arms with a π (180°) relative combining phase shift. A photonic sampling system described by Kanter [3] combines the interferometer arms with a 90° combining phase using an optical hybrid combiner also known as an in-phase quadrature-phase (I/Q) combiner. A variety of other optical samplers are known in the art [1].
In general one important performance issue with samplers are the requirements on the acceptable range the applied input signal. The applied signal size may be measured in volts or in dBm (decibels referenced to milliWatts). The dynamic range (DR) of a signal can be defined as the ratio of the largest acceptable input signal (with acceptable distortion) to the smallest measurable input signal. Ideally small signals can be measured and large signals can be tolerated leading to a large DR. Methods of having parallel ADCs have been proposed in order to increase the DR such as Rivera U.S. Pat. No. 5,111,202 (1992). U.S. Pat. No. 5,111,202 splits the input signal into multiple branches and sends it to multiple ADC units. The effective sensitivity and maximum acceptable input power of the multiple ADCs are different (one having a better sensitivity and one a higher maximum signal level), so by analyzing all the branches a higher overall DR can be achieved.
Optical samplers often use optical phase modulators to convert the input signal to be sampled from the electronic to the optical domain. In standard interferometric configurations phase modulators typically operate at input voltages less than the half-wave Vπ voltage (voltage required for a π phase shift) because 180° combiners are π periodic. This limits the maximum input signal, with the minimum signal limited by the smallest induced phase modulation that leads to a measureable signal. The I/Q hybrid technique can tolerate an input signal of up to 2·Vπ and accomplishes this without reducing sensitivity and therefore can have a 2× larger DR. Optical modulators often have relatively high Vπ voltage levels. For instance, a phase modulator constructed from Lithium Niobate such as the Covega Mach-10 Phase modulator may have a Vπ of >3 volts, while an electrical sampler such as the National Semiconductor ADC12D1000 can have a much lower full scale input signal of ˜0.6 V. Ideally the Vπ level would be smaller so that smaller signals can be observed thus improving signal sensitivity. Changing Vπ does not necessarily impact DR however, since both the minimum and the maximum tolerable signal changes in equal ratios. Regardless of the current state of the art, it is generally beneficial to lower the required signal size for a given level of performance as it makes the sampler more sensitive to the often small level signals that one may want to digitize and can either eliminate the need to amplify the electrical signal before sampling or at least reduce the output power or gain requirements of such an amplifier.
One technique that has been used in the art to allow a given electrical amplifier technology with a fixed maximum output voltage level to drive the high voltages required for optical modulators is to create push-pull modulators. Here dual phase modulators in different arms of an interferometer are driven to generate opposite phase shifts. This can be done for instance by using two separate phase modulators with input voltage signals of opposite polarity or by using domain inversion to allow a single voltage polarity to drive both modulators while still allowing for a push-pull action [4].
Nonlinear optical frequency conversion is used in various applications including for instance generating optical signals at a frequency of twice a given optical signal source, such as using an inexpensive infrared laser followed by a second harmonic generation harmonic generation (SHG) nonlinear crystal to generate green light. The process of SHG converts an electric field of Eoeiωt+φ to Eo′ei2ωt+2φ, thereby doubling its angular frequency ω and likewise doubling the inherent phase. Other nonlinear effects like parametric amplification can also be used to perform useful functions such as phase conjugation [5].
Photonic ADCs are sometimes operated in an under-sampling regime, where the sampling rate is less than twice the highest input signal frequency. In such a regime the input signal frequency is not determined unambiguously using traditional sampling theory.
What is needed is a system or method to reduce the size of the required driving signal to modulate an optical signal in order to improve sensitivity to small electrical driving signals, ideally while preserving or extending the maximum tolerable input signal size and thereby also improving the dynamic range. A push-pull configuration is desirable since it can in some cases lower the required drive voltage. Some phase shift technologies cannot be easily configured to apply the opposite phase shifts needed in a push-pull configuration when the phase shifters are driven by the same voltage polarities, so it is useful if the push-pull configuration can operate without this requirement. In such a case the system can be realized with single-ended drive signals (no differential electrical amplifiers or baluns required) and without the additional fabrication step of poling. It is useful if the system can be operated as a sampler, and moreover if the sampler can identify the input signal frequency over a large frequency range even when operating in an under-sampling regime. Ideally the system should be able to operate at high input frequencies such as frequencies of 10's of GHz or higher. These benefits should be realized with a minimum amount of drawbacks such as increased size, weight, or power consumption.